Sizing of the overpacks containing high-level radioactive waste packages from French nuclear facilities

This work is a contribution to the project Cigeo (Geological Storage Industrial Center: www.cigeo.com) which aims to store in a deep geological layer, intermediate, high-level and long-lived radioactive waste from all French nuclear facilities. The project is led by ANDRA (French National Radioactive Waste Management Agency). EDF (Electricity of France), as the nuclear electricity producer, will be a beneficiary of the facility and follows it for reasons of consistency with the rest of the nuclear cycle. Packages of the most radioactive vitrified waste from the processing of spent fuel will be placed in steel overpacks (storage containers) whose main function will be to protect the waste from water during the so-called thermal phase of the glass (t° glass < 50°C - 70°C). In addition to the waterproof criterion, the challenges associated with the design of the overpacks are: To limit the production of hydrogen due to corrosion (∼50.000 overpacks). To increase glass life: a decrease in steel thickness reduces the amount of corrosion products and improves the longevity of the glass. To reduce production costs. The objective of this work is to implement the radiological dimensioning methodology based on neutron modeling. As an illustration of what this method allows to do, a sizing of the steel overpack is proposed in order to achieve these technical and economic specifications. The overpack is mainly designed by taking into account the mechanical load that will be applied and degradation due to corrosion, particularly the corrosion by radiolysis. According to ANDRA, the most important criterion is to maintain the dose rate below 10 Gy/h at a distance of 5 cm from the overpack interface. The dose rates were calculated with the TRIPOLI4 code using as input data the temporal radiological evolution of waste obtained with DARWIN/PEPIN2. This work has shown that, in the most penalizing case among the studied configurations, a steel thickness of 4.6 cm for the overpack is sufficient to comply with the criterion of dose rate defined by ANDRA. Parametric studies have also shown a high sensitivity of results with the duration of prior packages' storage (cooling time before being sent to CIGEO) and to dose rate criterion. The isotope having the highest activity and generating the largest number of photons is 137Cs which contributes to more than 99% of the total dose rate over the whole period of reversibility at CIGEO (∼ 100 years). (author)